CT value in a Covid-19 test
- January 23, 2022
- Posted by: OptimizeIAS Team
- Category: DPN Topics
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CT value in a Covid-19 test
Subject – Science and Tech
Context – Among various scientific terms that the Covid-19 pandemic has made part of the public vocabulary, one is the ‘Ct value’ in RT-PCR tests for determining whether a patient is positive for Covid-19.
Concept –
- Short for cycle threshold, Ct is a value that emerges during RT-PCR tests, the gold standard for detection of the SARS-CoV-2 coronavirus. In an RT-PCR test, RNA is extracted from the swab collected from the patient. It is then converted into DNA, which is then amplified. Amplification refers to the process of creating multiple copies of the genetic material — in this case, DNA. This improves the ability of the test to detect the presence of virus.
- Amplification takes place through a series of cycles — one copy becomes two, two becomes four, and so on — and it is after multiple cycles that a detectable amount of virus is produced.
- According to the ICMR advisory, the Ct value of an RT-PCR reaction is the number of cycles at which fluorescence of the PCR product is detectable over and above the background signal.
- Put simply, the Ct value refers to the number of cycles after which the virus can be detected. If a higher number of cycles is required, it implies that the virus went undetected when the number of cycles was lower.
- The lower the Ct value, the higher the viral load — because the virus has been spotted after fewer cycles.
Why is CT value important?
- According to the ICMR, a patient is considered Covid-positive if the Ct value is below 35. In other words, if the virus is detectable after 35 cycles or earlier, then the patient is considered positive.
What is the significance of the ICMR threshold of 35?
- Globally, the accepted cut-off for Ct value for Covid-19 ranges between 35 and 40, depending on instructions from the respective manufacturers of testing equipment.
- The ICMR has arrived at the Ct value of 35 based on laboratory experiences and inputs taken from several virology labs.
Is there any correlation between a Ct value and severity of disease?
- Although Ct value is inversely correlated with viral load, It does not have any bearing on the severity of the disease, experts have said.
- A patient can have a low Ct value, which means her viral load is high enough to be detected rapidly, but she may still be asymptomatic.
- The Ct value tells us about the viral load in the throat and not in the lungs.
- The Ct value does not correlate with severity – only with infectivity.
Does a high Ct value always mean a low viral load?
- While that may be the obvious inference, some experts stress that some patients can have a high Ct value and yet have a very significant level of Covid-19 infection, and vice versa.
- Many factors are important in interpreting an RT-PCR test, and the results may also depend on the method of specimen collection and time from infection to collection and to analysis.
- Besides, Ct values are also determined by the technical competence of the person performing the test, calibration of the equipment, and the analytical skills of the interpreters.
- Again, Ct values may differ between nasal and oropharyngeal specimens collected from the same individual. The temperature of transportation, as well as the time taken from collection to receipt in the lab, can also adversely impact Ct values.